U.S. patent number 5,080,755 [Application Number 07/420,322] was granted by the patent office on 1992-01-14 for process for the continuous digestion of cellulosic fiber material.
This patent grant is currently assigned to Kamyr AB. Invention is credited to Ake Backlund.
United States Patent |
5,080,755 |
Backlund |
January 14, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Process for the continuous digestion of cellulosic fiber
material
Abstract
A process for the continuous digestion of cellulosic fiber
material is described wherein the material is impregnated with
liquid in a closed system comprising a concurrent flow zone and a
countercurrent flow zone, the liquid in the concurrent flow zone
including black liquor and possibly white liquor and the liquid in
the countercurrent flow zone including white liquor. Liquid is
withdrawn from the impregnation system at a point located between
the concurrent flow zone and the countercurrent flow zone. Besides
white liquor a predetermined amount of black liquor is added to the
countercurrent flow zone in order to obtain a high liquid to wood
ratio in the inlet of the digester.
Inventors: |
Backlund; Ake (Karlstad,
SE) |
Assignee: |
Kamyr AB (Karlstad,
SE)
|
Family
ID: |
20374278 |
Appl.
No.: |
07/420,322 |
Filed: |
October 12, 1989 |
Foreign Application Priority Data
|
|
|
|
|
Dec 20, 1988 [SE] |
|
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8804578 |
|
Current U.S.
Class: |
162/19; 162/41;
162/62; 162/242; 162/251; 162/39; 162/59; 162/239; 162/249 |
Current CPC
Class: |
D21C
11/0021 (20130101); D21C 1/00 (20130101) |
Current International
Class: |
D21C
11/00 (20060101); D21C 1/00 (20060101); D21C
003/26 (); D21C 007/12 () |
Field of
Search: |
;162/19,29,37,39,59,237,239,242,248,249,251,41,62 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Fisher; Richard V.
Assistant Examiner: Burns; Todd J.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
That which is claimed is:
1. A process for the continuous digestion of cellulosic fiber
material using a closed impregnation system having a concurrent
flow zone followed by a countercurrent flow zone, and a digester
system, comprising the steps of impregnating the fiber material
with liquid in the closed impregnation system, withdrawing a
predetermined amount of liquid from the impregnation system at a
point located between the concurrent flow zone and the
countercurrent flow zone, and supplying to the concurrent flow zone
black liquid, and to the countercurrent flow zone white liquor and
a predetermined amount of black liquor;
wherein the liquids withdrawn from the impregnation system and the
digester system are transferred to at least two serially connected
flash cyclones for heat recovery, the black liquor supplied to the
countercurrent flow zone of the impregnation system comprising
substantially the effluent from the first one of the flash cyclones
which receives liquid withdrawn from the digester between a
digester concurrent flow zone and a digester countercurrent flow
zone, and the black liquor supplied to the concurrent flow zone of
the impregnation system consisting of the effluent from a
subsequent flash cyclone which receives liquid withdrawn from the
digester via said first flash cyclone and liquid withdrawn from the
impregnation system.
2. A process as recited in claim 1 wherein the black liquor
supplied to the countercurrent flow zone of the impregnation system
also comprises liquid withdrawn from the digester which has not
passed the first flash cyclone.
3. A process as received in claim 1 comprising the further step of
supplying white liquor to the concurrent flow zone.
4. A process as recited in claim 3 wherein the impregnation system
includes an impregnation vessel and black liquor and white liquor
are added to the fiber material in the concurrent flow zone in an
amount such that the quantity of free liquor in the concurrent flow
zone of the impregnation vessel is above 0.5 m.sup.3 /ton dry fiber
material.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a process for the continuous
digestion of cellulosic fiber material.
Through U.S. Pat. No. 3,802,956 it is known that wood can be
impregnated with white liquor in countercurrent flow and that black
liquor can be added to the wood material at the inlet to the
impregnation vessel. The object of this procedure is primarily to
increase the concentration of active chemicals in the digesting
liquor by withdrawing a certain amount of impregnation liquid in
which the content of active chemicals has been substantially
consumed. The liquid to wood ratio in the digester is thereby
lowered, thus giving a high concentration of active chemicals which
results in rapid digestion. The smaller amount of liquid in the
digester in comparison with conventional methods also results in
less steam consumption, particularly high-pressure steam. However,
it has been found that a low liquid to wood ratio may entail
problems in controlling the cooking process, as well as
difficulties with the movement of the chip column due to
differences in relative speed between chips and free liquid. The
relatively high concentration of chemicals in alkaline digestion
processes also causes attack on the carbohydrates in the raw wood,
resulting in lowered pulp viscosity and pulp strength.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an improved
process for the continuous digestion of cellulosic fiber material
which eliminates the above-mentioned drawbacks of low liquid to
wood ratio in the digester and relatively high alkali concentration
at the beginning of the digestion.
The invention relates to a process for the continuous digestion of
cellulosic fiber material comprising the steps of impregnating the
fiber material with liquid in a closed system comprising a
concurrent flow zone and a countercurrent flow zone, withdrawing a
predetermined amount of liquid from said impregnation system at a
point located between the concurrent flow zone and the
countercurrent flow zone, and supplying to the concurrent flow zone
black liquor and possibly white liquor and to the countercurrent
flow zone white liquor and a predetermined amount of black liquor.
According to a preferred embodiment of the invention, the black
liquor is added to the countercurrent flow zone in such an amount
that a predetermined high liquid to wood ratio is obtained in the
inlet of the digester. This liquid to wood ratio is suitably 2.0 to
1 to 4.5 to 1, preferably 3.0 to 1 to 3.5 to 1.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described further in the following with
reference to the drawing showing schematically a flow diagram of a
plant for continuous digestion of fiber material impregnated in
accordance with the present invention.
DESCRIPTION OF ILLUSTRATED EMBODIMENT
The plant shown in the Figure comprises a horizontal steaming
vessel A, a vertical impregnation vessel B and a vertical digester
C. The disintegrated fiber material, preferably consisting of wood
chips, is fed from a chip bin 1 through a low-pressure valve 2 to
the steaming vessel A. Low-pressure steam, having a pressure of
e.g. 1 atmosphere over pressure, is supplied to the steaming vessel
A through a pipe 3 and air expelled is removed through a pipe 4.
After passing through the steaming vessel A for 2 to 5 minutes, the
chips fall down into a high-pressure valve 5 comprising a rotor
with pockets or diametrical channels, pivotable in a housing. From
there the chips are pumped up to the top of the impregnation vessel
B by means of a circulating liquid which is caused by a pump 6 to
flow through a supply pipe 7 and a return pipe 8. The liquid
flushes the chips from the high-pressure valve 5 and feeds the
chips in suspension through the supply pipe 7 to the top of the
impregnation vessel where a strainer (not shown}is disposed to
separate a certain portion of the liquid for recirculation. The
liquid strained off is returned through the return pipe 8 to the
high-pressure valve 5. The supply pipe 7 and return pipe 8 thus
form a circulation system for feeding liquid-carried chips.
The chips are fed into the pockets of the high-pressure valve 5 by
means of liquid circulated in a pipe 10 by a pump 9. Liquid which
is returned to the low-pressure side flows from this pipe 10 to a
level tank 11 connected to the top of the impregnation vessel B via
a pipe 12 to feed back the liquid to the high-pressure side by
means of a pump 13 disposed in the pipe 12. The circulation pipe 10
is connected to a chip feeder 14 before the high-pressure valve 5
via a sand separator 15 and a pair of screens 16 for screening off
excess liquid. Sand and similar undesired particles are removed
from the sand separator 15 through a pipe 17.
The impregnation vessel B consists of a vertical, elongate
container with circular cross section, suitably becoming wider
towards the bottom. The impregnation vessel constitutes or forms a
part of a closed impregnation system which, in the embodiment
shown, consists of a concurrent flow zone 52 and a countercurrent
flow zone 53. At the bottom of the impregnation vessel is a device
(not shown) for continuously feeding out chips which have been
impregnated with supplied liquids as they move continuously
downwards. The impregnation vessel B is provided with a strainer 18
disposed in the wall of the vessel for the removal of a
predetermined amount of liquid Q.sub.A from the chip suspension.
The liquid withdrawn through the strainer 18 is passed through a
pipe 19 to the second one of two flash cyclones 21, 22 connected in
series and joined to each other by a pipe 20.
A specified amount of black liquor is pumped through pipe 12 to the
top of the impregnation vessel B, the black liquor being supplied
through a pipe 23 from the second flash cyclone 22. If desired a
small amount of white liquor may be added at the top of the
impregnation vessel through a pipe 24, branch pipe 25 and pipe
12.
The impregnated chips are transferred from the bottom of the
impregnation vessel B to the top of the digester C by liquid, i.e.
digesting liquor, through a supply pipe 26 connected to an outlet
28 at the bottom of the impregnation vessel. A strainer (not shown)
is disposed at the top of the digester to separate a certain
portion of the liquid for recirculation. The circulation liquid is
returned through a return pipe 27 provided with a pump 29, such a
strong liquid flow being maintained by the pump in the pipes 26, 27
that chips are carried with it and flushed out through the outlet
28. The supply pipe 26 and return pipe 27 thus form a transfer
circulation system for the suspension of impregnated chips and
digesting liquor.
In order to achieve uniform distribution of the alkali flowing in
countercurrent flow, and to offer the best possible conditions for
reaction between alkali and wood, a strainer 47 is preferably
inserted at a place between the strainer 18 and the bottom of the
impregnation vessel. An amount of liquid is removed from this
strainer 47 and circulated through a pipe 48 and pump 29 to the
bottom of the impregnation vessel. The countercurrent flow in the
lower portion of the countercurrent flow zone will therefore be
greater than the upward flow in the upper portion of the
countercurrent flow zone located above the strainer 47.
Most of the heating of digesting liquor and wood material occurs
indirectly by the addition of high-pressure steam through a pipe 33
to a heat exchanger 34 in the return pipe 27 through which the
circulating digesting liquor flows. This heating causes increased
reaction rate between wood and effective alkali in the
countercurrent flow zone.
The digester is provided with a strainer 30 for circulation of
liquid through a pipe 31 by means of a pump 32, the liquid being
heated in a heat exchanger 55. The pipe 31 contains a central pipe
disposed at the centre of the digester and having its orifice at
the strainer 30. The digested fiber material is washed in
countercurrent flow in the lower part of the digester, using a
washing liquid supplied through a pipe 35 and pumped by a pump 36
into the lower end of the digester in an amount adjusted in such a
manner that the digester is kept filled with liquid. The washing
liquid is heated indirectly by steam supplied to a heat exchanger
37 disposed in a pipe 38 for circulation of washing liquid by a
pump 39. The washing liquid is withdrawn through a strainer 40 and
returned through a central pipe extending from the bottom of the
digester to the strainer 40. The washing liquid heated in this way
is forced upwardly in countercurrent flow through the chips column
which is slowly moving downwards, and thereby displaces its content
of spent digesting liquor. This can then be withdrawn through a
strainer 41 and passed via a pipe 42 to the first one of the two
flash cyclones 21, 22. Below the strainer 41 is another strainer 49
for circulation of liquid through a pipe 50 by a pump 51 disposed
therein, the liquid being circulated via a central pipe having its
orifice at the strainer 49. Effluent from the second flash cyclone
22, which is not supplied to the impregnation vessel, is passed
through a pipe 56 to a recovery plant. The digested fiber material
is discharged at the bottom of the digester by a suitable scraping
device and is passed through a pipe 57 for continued treatment.
Besides the digesting liquor and wood being indirectly heated in
said transfer circulation system 26, 27, they are also directly
heated by steam supplied to the top of the digester through a pipe
44.
The strainer 18 in the impregnation vessel B is so located that
sufficient retention time is obtained for concurrent flow
impregnation with black liquor and possibly a small amount of white
liquor. The distance to the bottom of the vessel is such that
sufficient retention time is obtained for countercurrent flow
impregnation with white liquor. For instance, suitable retention
times may be 10-20 minutes for concurrent flow impregnation with
black liquor and 10-20 minutes for countercurrent flow impregnation
with white liquor.
The total amount of liquid to the top of the impregnation vessel B,
including chips moisture, steam condensate, black liquor and any
white liquor, shall be sufficient to completely saturate the chips
with liquid and also to give a certain excess of non-bound liquid
in the chips. The bound liquid Q.sub.B in the chips is for pine 1.8
and for birch 1.3 m.sup.3 /ton dry wood. The amount of free liquid
Q.sub.F supplied to the top of the impregnation vessel should not
be less than 0.5m.sup.3 /ton dry wood. In order to improve the flow
conditions for the chips, the amount of free liquid Q.sub.F may
advantageously be increased to 1.0 m.sup.3 /ton dry wood, and under
certain conditions, up to 2.5 m.sup.3 /ton dry wood or higher. (The
expression "dry" refers to bone dry in the present
specification).
An amount of liquid Q.sub.A, which would be greater than the amount
of free liquid Q.sub.F in the upper part of the impregnation
vessel, is withdrawn from the strainer 18. The difference would be
so great that an upward flow from the bottom of the impregnation
vessel encounters the descending chips and that effective alkali in
the white liquor being drawn upwards is consumed through reaction
with the wood material. The upward flow should be limited so that
the content of effective alkali remaining in the liquid Q.sub.A
withdrawn is approximately equivalent to the content of alkali
remaining in the liquor withdrawn from the digester for the
chemical recovery via the strainer 41, pipes 42, 20, 23 and a pipe
54.
An amount of white liquor required for carrying out the digestion
is supplied to the bottom of the impregnation vessel B through a
pipe 45 which connects the pipe 24 with the return pipe 27. With a
normal white liquor concentration, this amount will be
0.8-1.6m.sup.3 /ton dry wood, depending on how great a portion of
the white liquor that is supplied to the wood at the top of the
impregnation vessel through pipes 25 and 12, the concentration of
effective alkali in the white liquor, and the amount of alkali
consumed by the wood. According to the present invention a specific
amount of black liquor is supplied together with the white liquor,
said black liquor being supplied from the flash cyclone 21 through
a pipe 46. The amount of black liquor is adjusted so that the
desired liquid to wood ratio is obtained in the concurrent flow
zone of the digester. This ratio is normally 2.0 to 1 to 4.5 to 1,
but in certain cases the liquid amount may be less than 2.0 ton/ton
dry wood or higher than 4.5 ton/ton dry wood.
Liquid to wood ratio means the total amount of liquid consisting of
wood moisture+steam condensate+white liquor+black liquor per ton
dry wood.
The temperature in the top of the impregnation vessel is generally
about 110-120.degree. C. and in its bottom, i.e. in the transfer
circulation system 26, 27, about 130-160.degree. C. The liquor
withdrawn through the strainer 18 has a temperature of about
120-135.degree. C. while the black liquor withdrawn from the
digester through the strainer 41 has a temperature of about
150-170.degree. C. A portion of the thermal content in the two
withdrawals or black liquors from the impregnation vessel and the
digester is recovered from the two flash cyclones 21, 22 and the
black liquor effluent from the first flash cyclone 21 may have a
temperature of e.g. 125.degree. C. while the black liquor effluent
from the second flash cyclone 22 may have a temperature of e.g.
102.degree. C. Black liquors can thus be returned from the two
flash cyclones 21, 22 to the process with a heat content close to
the temperatures which would be maintained at the top and bottom,
respectively, of the impregnation vessel. This has a great value
from the thermal economy point of view. It is naturally possible to
supply black liquor to the bottom of the impregnation vessel which
consists partially or completely of liquor withdrawn from the
digester. Said withdrawn liquor may be added in particular if it is
advantageous from the thermal economy point of view. For this
purpose a connection 54 is disposed between pipes 42 and 46.
An example is given below of cooking pine in accordance with the
invention. Using the designations in the Figure and below, the
total amount of liquid Q.sub.T per ton of dry wood is calculated in
the concurrent flow zone of the digester according to the following
equation:
The amounts of liquid per ton of dry wood are as follows:
______________________________________ Chips moisture 1.0 m.sup.3
Steam condensate to steaming vessel 0.3 m.sup.3 White liquor to top
of impregnation vessel 0.4 m.sup.3 Black liquor to top of
impregnation vessel 1.5 m.sup.3 Total amount of liquid in
concurrect flow 3.2 m.sup.3 zone of impregnation vessel Bound
liquid in chips (wood density Q.sub.B = 1.8 m.sup.3 0.40
ton/m.sup.3) Free liquid in concurrent flow zone of Q.sub.F = 1.4
m.sup.3 impregnation vessel (3.2-Q.sub.B) Withdrawn liquid from
strainer in Q.sub.A = 2.0 m.sup.3 impregnation vessel Upward flow
in countercurrent flow Q.sub.A -Q.sub.F = 0.6 m.sup.3 zone of
impregnation vessel White liquor to bottom of Q.sub.V = 1.2 m.sup.3
impregnation vessel Steam condensate to top of digester Q.sub.C =
0.2 m.sup.3 ______________________________________
In order to achieve a liquid to wood ratio of 3.2 to 1 in the
digester, the amount of black liquid Q.sub.S which must be supplied
to the bottom of the impregnation vessel is calculated according to
the following equation:
The balance ratio for effective alkali as NaOH is approximately as
follows for the two additions of white liquor:
______________________________________ Effective alkali to top of
45 kg NaOH/ton dry wood impregnation vessel Effective alkali to
bottom of 135 kg NaOH/ton dry wood impregnation vessel Total charge
of effective alkali 180 kg NaOH/ton dry wood
______________________________________
Consumption of effective alkali in the impregnation vessel B is
distributed as follows:
______________________________________ In concurrent flow zone 52
40 kg NaOH/ton dry wood In countercurrent flow zone 53 50 kg
NaOH/ton dry wood Total consumption in 90 kg NaOH/ton dry wood
impregnation vessel ______________________________________
The liquid Q.sub.A withdrawn from the impregnation vessel contains
effective alkali in an amount of 15 kg NaOH/ton dry wood. The
remaining effective alkali conveyed to the digester will therefore
be 180-90-15=75 kg NaOH/ton dry wood, corresponding to a
concentration of effective alkali at the beginning of the cooking
zone of the digester of 75/3.2=23 g NaOH/l digesting liquor.
The concentration of effective alkali obtained, 23 g/l calculated
as NaOH, is sufficiently low not to cause any appreciable breakdown
of the carbohydrates of the pulp during the initial stage of the
digestion. Should an even lower concentration be desired, this can
be provided by passing a flow of liquid from the trimming strainer
30 in the digester to the transfer circulation system. Due to the
consumption of effective alkali in the upper portion of the
digester, the concentration of effective alkali in the trimming
circulation system through the strainer 30 will be lower than in
the feed-back of the transfer circulation system. The content of
effective alkali in the transfer circulation system is thereby
further lowered.
The process according to the invention can also be utilized in
two-vessel hydraulic digesters where the liquid in the transfer
circulation system is heated to full cooking temperature, i.e.
160-170.degree. C.
In the embodiment shown in the Figure impregnation is combined with
concurrent flow cooking in the digester C. It is also highly
beneficial in extended digestion where cooking is also performed in
two stages comprising a first concurrent flow stage and a second
countercurrent flow stage.
The process according to the invention is also applicable in
continuous operating digesters where impregnation and cooking are
carried out in the same vessel, the impregnation stage being
performed in the upper part of the vessel and the cooking stage
therebelow.
* * * * *